Recent industry interest in the degradation potential of cigarette filters has prompted a research effort to improve the environmental degradation of cellulose acetate (CA) fibers. Biodegradation studies indicated that lowering the degree of substitution per anhydroglucose unit (DS/AGU) of the cellulose acetate below a level of 2.5 will result in an increase in the biodegradation rate of the cellulose acetate. However, the ability to spin such a polymer is problematic because of its modified solubility parameters.
The presence of small amounts of water in acetone/CA dopes is well known to the industry. Much of the water found in conventional acetone/CA dopes is the residual left in the incompletely dried CA flake and in the acetone solvent used to make the dopes. These water levels are controlled because it is recognized that variation in this level does have an affect on dope viscosity. For example, the use of small amounts of water (95:5 acetone/water) in acetone dopes of 25% CA with a DS of 2.4 to reduce the viscosity of the solution is described by H. W. Steinman in the Handbook of Fiber Science and Technology: Fiber Chemistry (Menachem Lewin and Eli M. Pearce Ed.) vol. 5. Marcel Dekker, New York, p. 1025.
The connection between the solubility of CA in acetone and the degree of substitution of the CA has also been studied. The Chemistry of Cellulose, by Emil Heuser, John Wiley & Sons, Inc., New York, p. 267-270, states that the "acetone-solubility range of commercial secondary acetates comprises an acetyl content between 35.8% and 41.5%" which corresponds to a range of DS of about 2.1 to 2.7. This reference discusses the variables affecting the solubility of cellulose acetate and identifies degree of polymerization, physical form, degree of substitution, and solvent type as the major variables. This reference also notes that if the degree of polymerization and physical form is neglected, the most satisfactory explanation for solubility or lack of solubility of CA is based on the relative amounts of polar and nonpolar groups in the solvent and the CA. Also noted therein are the results of a number of studies done during the 1920's and 1930's on the solubility of CA and the affect of water on this solubility.
In "Colloid Symposium Monograph V", by Sheppard, Carver, and Houck, 243 (1928), the authors note: "maximum solvent power when a certain quantity of water had been added to the solvent". In Sheppard and Sweet, J. Phys. Chem., 36, 819 (1932), the authors note: "when a certain quantity of a non-solvent was added to an acetone acetate solution, a point was established at which the nonsolvent developed solvent properties in conjunction with the acetone". (See also, Whitby, "Colloid Symposium Monograph IV", 203 (1926)). In Werner and Engelmann, Z. Angew. Chem., 42,443 (1929), the authors note "acetone-insoluble acetate (containing 50 percent combined acetic acid) was soluble in a mixture of acetone and alcohol or acetone and water".
European Patent Application 597 478, (in Example 6) describes using 2.14 DS cellulose acetate to produce a 5 denier per filament fiber by dry spinning. This reference shows the preparation of formulated spinning dope by dissolving the low DS cellulose acetate in a 96.5/3.5 by weight mixed solvent of acetone and water. The actual water level in the DOPE solution works out to about 2.5 weight percent.
However, none of the prior art makes the connection between the successful dry spinning of low DS cellulose acetate fibers and the importance of higher than normal levels (5 weight percent or more) of water in the CA/acetone dopes. They also do not recognize that the percentage of water in the DOPE solution is the most important factor. The prior art references generally quote an acetone/water ratio in the solvent and do not take into account the fact that as CA is added to the solvent the percentage of water relative to the total solution is reduced.